This invention relates to an illumination system for a document scanning system and, more particularly to a fluorescent lamp and heating assembly which is thermally stabilized to emit an output in which temporal variations in axial light output along its length are minimized.
Low pressure, mercury vapor fluorescent lamps are used in a variety of lighting applications. Of particular interest, for purposes of the present invention, is the widespread use of fluorescent lamps to illuminate documents being copied by an electrophotographic reproduction machine.
In a conventional mercury fluorescent lamp, an electrical discharge is generated in a mixture of mercury vapor, at low pressure, and a fill gas typically argon, neon, Krypton, xenon or mixtures thereof.
The temporal stability of the axial light output from the lamp, referred to as the lamp illumination profile, depends on a number of variables, the most important of which is the mercury vapor pressure within the lamp. The internal aggregate mercury vapor pressure is controlled by the coldest spot in the lamp referred to as the cold spot. Spatial illumination changes occur as mercury responds to localized temperature changes by moving back and forth between the cold spot and the internal walls of the lamp. In addition to the cold spot temperature, there are other operating conditions which cause a fluorescent lamp to exhibit a large and unpredictable temporal and spatial variability in its illumination output profile. Factors contributing to the instability directly or indirectly influence mercury pressure stability and are caused by: temporal variations in the cold spot temperatures, changes in the mercury/phosphor attachment along the internal walls of the lamp envelope during periods when only the filaments are energized, such periods being known as standby periods; and changes in operating mode when changing from a standby period to full energization. These factors become more significant when the lamp is operated at high power loadings where the lamp temperature change between standby and run is large. When the lamp is used in a document copying application, where a uniform band of illumination is required to scan a document to be reproduced, non-uniform light output from the temporal variations in light output along the length of the lamp results, ultimately, in defects in the output copies produced following the xerographic steps of exposure, development, transfer and fusing.
Fluorescent lamp instability is especially acute in applications which require the use of Extra High Output (EHO) lamps to produce a high output illumination level. For purpose of this invention such lamps are defined as lamps drawing approximately two amps of current. Lamps in this range find typical application in high throughput raster input scan (RIS) systems where the lamp is used to illuminate a document on a platen and the reflected line images are captured electronically at an image plane comprising a linear array of photoreceptors, such as charge-coupled diodes (CCD). The invention, however, is not limited only to EHO lamps used in electronic scanning applications, but will also find utility in conventional light lens copiers which require a fluorescent lamp as the illuminator.
It is known in the art to stabilize the operation of fluorescent lamps by wrapping a heating jacket around the lamp and maintaining the jacket at a desired temperature above that of the cold spot. U.S. Pat. No. 4,751,551 discloses a typical lamp assembly having heater jacket 8 wrapped around most of its length. It is also known in the art to use a segmented jacket with independently powered sections to produce temperature gradients along the entire length as, for example, in U.S. Pat. No. 4,827,313, and Japanese Publication 59-42534. These prior art lamp/jacket assemblies while providing some compensation for lamp profile variations may not prove effective and for all systems for all environmental and temperature conditions. A recurring difficulty when using fluorescent lamps is the problem of illumination changes when changing between the standby and full-on active states. Following a normal operation, and a return to standby, the mercury in the lamp tends to stick to the interior walls of the lamp rather than return to the desired cold spot location. The greater the difference between the operating temperature profile and the lower standby temperature profile, the greater is the tendency for the mercury concentration at the envelope walls and in the discharge area of the lamp envelope to change. These changes correlate directly with the deleterious time-varying changes in the illumination output of the lamp. While in the prior art, heat is supplied by a heater blanket to maintain the lamp above the temperature of the cold spot in standby, there is no provision to insure that the heater blanket does not turn off when the lamp is fully powered up since in the power-on mode, the lamp typically runs much hotter than in standby. This is especially true for high power lamps. In such cases the temperature exceeds the blanket temperature control point and electrical energy to the blanket is shut off. This produces a significant difference in the temperature profile along the lamp wall between the standby and run operating states. In standby mode, the profile and upper temperature is defined by the blanket; in the run mode by the lamp. There is a time varying temperature change during critical periods when changing between the standby and power-on states. The resulting changes in lamp wall temperature causes changes in localized mercury vapor pressure and a corresponding change in localized illumination.
It is, therefore, desirable to provide a fluorescent lamp with a heater blanket which maintains the same constant lamp temperature profile during both the lamp operational cycle and also during standby. It is equally desirable to control the temperature to the extreme ends of the lamp including the areas adjacent to the filaments.
According to a first aspect of the present invention, an apertured fluorescent lamp is wrapped from one end to the other with a heater blanket incorporating a uniform resistive grid pattern. Since the blanket is a unitary piece, it is easily and mechanically secured to the lamp envelope. Compensation for the environmental temperature changes along the length of the lamp adjacent to the end filaments is achieved by providing a bifurcated heater control resulting in independent application of heat for the right and left sides of the lamp measured from the center point. Compensation for the increased lamp temperature load caused by the filaments is provided by a separate thermostat located on the heater blanket over the filament zones which is designed to shunt the current and therefore, the heat being applied to the heater blanket adjacent the filament areas in an over temperature condition. Use of a shunt rather than a separately powered heater element allows localized over temperature control of the extreme ends of the heater blanket without a separate or special power source to power the small end segments. The need for a special or separate low voltage power source is then eliminated even though the small heaters at the extreme ends of the lamp are necessarily short and of low resistance in an etched foil resistive heater blanket. According to another aspect of the present invention the heater blanket is first wrapped around a sheet barrier layer such as aluminum which slows thermal transients and prevents excessive localized heating which can otherwise result in delamination and thermal changes to the blanket at power up.
More precisely the invention relates to a fluorescent lamp assembly comprising, in combination, an elongated fluorescent lamp containing an excess of mercury at a cold spot therein, said lamp adapted, when energized by a power source, to emit light radiation through a clear apertured slit, a resistive heater blanket attached to the surface of the lamp, and means for controlling power to said heater blanket so as to maintain a uniform end-to-end temperature distribution of said lamp whether said lamp is in a standby or energized state.